Drainage water containing silicon powders which will become waste, more specifically drainage water containing fine metal silicon or silicon chips is discharged from the following places.
For example, metal silicon is used in the manufacture of silicon chloride which is a raw material of silicon to be reacted with hydrogen chloride. When this silicon chloride is collected, drainage water containing fine metal silicon used for the reaction is discharged. Since polycrystal silicon rods, monocrystal silicon rods and silicon wafers all of which are obtained from the above silicon chloride become products through cutting and polishing steps, drainage water containing silicon chips is discharged. Since silicon powders contained in this silicon powder-containing drainage water containing metal silicon or silicon chips are very fine particles (especially silicon chips), a coagulation treatment is difficult and various treating methods are proposed.
For example, there is proposed a method in which drainage water containing metal silicon chips is treated by adding an inorganic flocculant such as aluminum chloride and adjusting its pH to 10 or more so as to suspend a metal silicon agglomerate (refer to JP-A 10-323675). Although the above method is excellent because the concentration of the residual chips in the finally obtained water becomes low, as pH is adjusted to 10 or more, hydrogen may be generated through a reaction between the metal silicon powder and water. Therefore, there is room for the improvement of operation efficiency.
Further, there are proposed a method in which silicon powder-containing drainage water is treated with pressurized water containing fine air bubbles by adding an inorganic flocculant, a floating aid and a polymer flocculant (JP-B 3-14516) and a method in which silicon powder-containing drainage water is treated by adding a pH control agent or antioxidant and further adding an inorganic flocculant (JP-A 2004-261708).
However, in the method disclosed by JP-B 3-14516, since the drainage water is treated with pressurized water containing fine air bubbles, the process becomes complicated and there is room for the improvement of operation efficiency. In the method disclosed by JP-A 2004-261708, the sedimentation rate of the obtained agglomerate is low and the turbidity of the finally obtained water cannot be reduced fully. Therefore, there is room for improvement.
Further, JP-B 4-75796 discloses a water treating flocculant composed of a silica sol-metal salt. However, in the method disclosed by JP-B 4-75796, the treatment of only water having a given turbidity is carried out and the method of treating silicon powder-containing drainage water which is the target of the present invention is not disclosed.